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Morning Mercury

The Messenger spacecraft captured a sliver of the southern hemisphere of Mercury in this recent image. Although Mercury is relatively close to Earth, it stays so near the Sun in our sky that it is difficult to study with ground-based telescopes, which reveal almost no surface features. In fact, astronomers couldn't even measure the length of Mercury's day until they began using radio telescopes in the 1960s. Even today, the only way to get a clear view of Mercury is from a spacecraft like Messenger, which is the first probe ever to orbit the Sun's closest planet. [NASA/JHUAPL/CIW]

As recently as the 1960s, astronomy textbooks reported that the length of a day and a year on the planet Mercury were the same. That’s because astronomers thought the same side of the planet always faced the Sun, just as the same side of the Moon always faces Earth. That would create a world where one hemisphere broiled in constant sunlight, while the other froze in perpetual night.

It turns out that that’s not the case, though. Mercury completes three turns on its axis for every two orbits around the Sun. So, on average, one “day” on Mercury lasts almost six Earth months.

The mistake came about because Mercury is difficult to study. The planet is small, and it’s quite close to the Sun in our sky, so it’s hard to see much detail on its surface. Every time Mercury came closest to Earth, astronomers saw the same vague features, so they concluded that the same side always faces the Sun. Instead, though, Mercury completes three turns on its axis between each closest approach to Earth, so the same features rotate into view each time Mercury comes close enough to see.

Astronomers discovered the truth about Mercury in the early ’60s, when they used radar to measure how fast the planet spins.

Look for Mercury quite low in the east shortly before sunrise the next few mornings. The planet looks like a fairly bright star. You need a clear horizon to see it, though, because any nearby trees or buildings will block it from view.

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